Flexible Merge Scheme for Source/Drain Epitaxy Regions

ABSTRACT

A method includes etching a first semiconductor fin and a second semiconductor fin to form first recesses. The first and the second semiconductor fins have a first distance. A third semiconductor fin and a fourth semiconductor fin are etched to form second recesses. The third and the fourth semiconductor fins have a second distance equal to or smaller than the first distance. An epitaxy is performed to simultaneously grow first epitaxy semiconductor regions from the first recesses and second epitaxy semiconductor regions from the second recesses. The first epitaxy semiconductor regions are merged with each other, and the second epitaxy semiconductor regions are separated from each other.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No. 15/492,142, entitled “Flexible Merge Scheme for Source/Drain Epitaxy Regions,” filed on Apr. 20, 2017, which application is incorporated herein by reference

BACKGROUND

Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. For example, three-dimensional transistors such as a Fin Field-Effect Transistors (FinFETs) have been introduced to replace planar transistors. Although existing FinFET devices and methods of fabricating FinFET devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. For example, the FinFETs for different circuits such as core (logic) circuits and Static Random Access Memory (SRAM) circuits may have different designs, and the source/drain epitaxy regions grown from neighboring fins may need to be merged for some circuits (such as logic circuits), and need to be separated from each other for other circuits (such as SRAM circuits). However, to save manufacturing cost, the epitaxy for different regions is performed simultaneously. This causes difficulty for selectively making epitaxy regions merged for some circuits, and not merged for other circuits. Accordingly, the merged epitaxy regions need to be trimmed to separate the merged epitaxy regions from each other.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A through 10C are cross-sectional views and perspective views of intermediate stages in the formation of Fin Field-Effect Transistors (FinFETs) in accordance with some embodiments.

FIG. 11 illustrates the exemplary layouts of a logic circuit and a Static Random Access Memory (SRAM) circuit in accordance with some embodiments.

FIG. 12 illustrates a process flow for forming FinFETs in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,” “lower,” “overlying,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Fin Field-Effect Transistor (FinFETs) and the method of forming the same are provided in accordance with various exemplary embodiments. The intermediate stages in the formation of the FinFETs are illustrated. The variations of some embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.

FIGS. 1A through 10C illustrate the intermediate stages in the formation of FinFETs. The steps shown in FIG. 1A through 10C are also illustrated schematically in the process flow 500 shown in FIG. 12. Each of the figure numbers in FIGS. 1A through 10C may include letter “A,” “B,” or “C,” wherein letter “A” indicates that the respective figure illustrates a perspective view, and letter “B” indicates that the respective figure is obtained from the plane same as the vertical plane containing line B-B in FIG. 1A, and letter “C” indicates that the respective figure is obtained from (and combined) the planes same as the vertical planes containing lines C-C in FIG. 1A. Accordingly, the figures whose numbers include letter “B” show the cross-sectional views obtained from the vertical planes parallel to the lengthwise directions of gate stacks, and the figures whose numbers include letter “C” show the cross-sectional views obtained from the vertical planes parallel to the lengthwise directions of semiconductor fins, which will be discussed in detail in subsequent paragraphs.

FIG. 1A illustrates a perspective view in the formation of a structure including substrate 20, isolation regions 22, semiconductor strips 24 between isolation regions 22, and semiconductor fins 26 over the top surfaces of isolation regions 22. Substrate 20 is a semiconductor substrate, which may be a silicon substrate, a silicon carbon substrate, or a substrate formed of other semiconductor materials such as III-V compound semiconductor materials. Substrate 20 may be lightly doped with a p-type or an n-type impurity.

Isolation regions 22 may be, for example, Shallow Trench Isolation (STI) regions. The formation of STI regions 22 may include etching semiconductor substrate 20 to form trenches, and filling the trenches with a dielectric material(s) to form STI regions 22. STI regions 22 may include silicon oxide, and other dielectric materials such as nitrides may also be used. Semiconductor fins 26 overlap the underlying semiconductor strips 24. The formation of semiconductor fins 26 may include recessing STI regions 22, so that the portions of semiconductor material between the removed portions of STI regions 22 become semiconductor fins 26. Semiconductor fins 26 and some or substantially entireties of semiconductor strips 24 may be formed of silicon (with no germanium therein) or other silicon-containing compound including, and not limited to, silicon carbon, silicon germanium, or the like.

A plurality of parallel gate stacks 28 is formed on semiconductor fins 26. Gate stacks 28 are parallel to each other, and cover portions of semiconductor fins 26, while leaving some other portions of semiconductor fins 26 uncovered. Gate stacks 28 include gate dielectrics 32 on the sidewalls and the top surfaces of semiconductor fins 26, and gate electrodes 34 over gate dielectrics 32. Gate dielectrics 32 may be selected from silicon oxide, silicon nitride, gallium oxide, aluminum oxide, scandium oxide, zirconium oxide, lanthanum oxide, hafnium oxide, combinations thereof, and multi-layers thereof. Gate electrodes 34 may be formed of a conductive material that includes polysilicon, a refractory metal, or the respective compound including, e.g., polysilicon, Ti, W, TiAl, TaC, TaCN, TaAlC, TaAlCN, TiN, and TiW. In other examples, gate electrodes 34 include nickel (Ni), gold (Au), copper (Cu), or the alloys thereof.

In accordance with some embodiments of the present disclosure, gate stacks 28 remain in the final FinFETs, and form the gate stacks of the final FinFETs. In accordance with alternative embodiments of the present disclosure, gate stacks 28 are dummy gate stacks that will be replaced by replacement gates in subsequent steps. Accordingly, gate stacks 28 may include dummy gate electrodes (which is also denoted as 34), which may comprise polysilicon, for example. Dummy gate dielectrics 32 may, or may not, be formed between dummy gate electrodes 34 and semiconductor fins 26.

Gate stacks 28 may also include hard masks 35 and 36 formed over gate electrodes 34. In accordance with some embodiments, hard masks 35 are formed of silicon oxide, silicon oxycarbo-nitride (SiOCN), or the like. Hard masks 36 may be formed of silicon nitride (SiN), SiOCN, SiOC, or other dielectric materials in accordance with some embodiments.

The perspective view show in FIG. 1A illustrates the exemplary layouts of circuits 602 and 604 as schematically represented by dashed boxes. In accordance with some embodiments, each of circuits 602 and 604 is selected from logic circuits or SRAM circuits, and circuits 602 and 604 may be same types of circuits or different types of circuits. In the following discussion, circuits 602 and 604 are referred to as a logic circuit and an SRAM circuit, respectively as an example, while other combinations are also contemplated.

In accordance with some embodiments, circuit 602 is formed in a device region including n-type FinFET region 100 and p-type FinFET region 200, and circuit 604 is formed in a device region including n-type FinFET region 300 and p-type FinFET region 400. Device regions 100, 200, 300, and 400 are also illustrated in FIGS. 1B and 1C through 10C. Semiconductor fins 126, 226, 326, and 426 are formed in regions 100, 200, 300, and 400, respectively, and are referred to collectively as semiconductor fins 26. Gate stacks 28 are formed in the directions perpendicular to the lengthwise directions of semiconductor fins 26. It is noted that although the gate stacks 28 are illustrated as continuously extending into different device regions 100, 200, 300, and 400 for a compact illustration purpose, the gate stacks 28 in different device regions may be physically separated from each other, or some gate stacks 28 in some of device regions may be connected in any combination, while the gate stacks 28 in other device regions are separated.

FIG. 1B illustrates the cross-sectional views of semiconductor fins 26 in device regions 100, 200, 300, and 400, wherein the cross-sectional view is obtained from the plane crossing line B-B in FIG. 1A. Also, the plane of cross-sectional view is obtained from the middle of two neighboring gate stacks 28 (as schematically illustrated in FIG. 1C). As shown in FIG. 1B, distance D1 between neighboring fins 126 may be greater than, equal to, or smaller than, distance D1′ between neighboring fins 326. Distance D2 between neighboring fins 226 may be greater than, equal to, or smaller than, distance D2′ between neighboring fins 426. The illustrated view in FIG. 1B reflects the structures shown in the regions marked by dashed lines 602 and 604 in FIG. 1A (also refer to FIG. 11).

FIG. 1C illustrates the cross-sectional views of device regions 100, 200, 300, and 400, wherein the cross-sectional views are obtained from the planes crossing lines C-C in FIG. 1A.

As shown in FIGS. 1A, 1B, and 1C, dielectric layer 38 is formed. The respective step is illustrated as step 502 in the process flow shown in FIG. 12. Dielectric layer 38 is alternatively referred to as a spacer layer. In accordance with some embodiments of the present disclosure, spacer layer 38 is formed of silicon nitride, silicon oxide, silicon carbo-nitride (SiCN), silicon oxy-carbo-nitride (SiOCN), silicon oxynitride (SiON), while other dielectric materials may be used. Spacer layer 38 may have a thickness in the range between about 2 nm and about 5 nm.

Spacer layer 38 is formed as a conformal layer, and hence covers the top surfaces and the sidewalls of semiconductor fins 26 (FIG. 1B) and gate stacks 28 (FIG. 1C). The portions of spacer layer 38 on the sidewalls of semiconductor fins 26 are used to form fin spacers, as shown in FIG. 2B, and the portions of spacer layer 38 on the sidewalls of semiconductor fins 26 are used to form gate spacers.

Mask layer 40 is formed over spacer layer 38. The respective step is also illustrated as step 502 in the process flow shown in FIG. 12. The material of mask layer 40 is selected to have a high etching selectivity with relative to the material of spacer layer 38. In accordance with some embodiments of the present disclosure, the material of mask layer 40 is also selected from silicon nitride, silicon oxide, silicon carbo-nitride (SiCN), silicon oxy-carbo-nitride (SiOCN), and silicon oxynitride (SiON). Mask layer 40 may have a thickness in the range between about 2 nm and about 10 nm. Mask layer 40 is also formed as a conformal layer. The formation of spacer layer 38 and mask layer 40 may be selected from conformal deposition methods such as Atomic Layer Deposition (ALD) and Chemical Vapor Deposition (CVD). Both spacer layer 38 and mask layer 40 extend into device regions 100, 200, 300, and 400.

FIGS. 2A, 2B, and 2C illustrate the patterning of spacer layer 38 in region 100. First, photo resist 42 is applied and patterned, wherein the photo resist 42 is illustrated in FIGS. 2B and 2C, and not in FIGS. 2A, although it still exists in FIG. 2A. Photo resist 42 may be a single-layer photo resist, or a tri-layer photo resist including an inorganic layer (known as middle layer) sandwiched between two photo resists (known as under layer and upper layer). The patterned photo resist 42 covers regions 200, 300, and 400, and leaves region 100 uncovered. Next, an etching step is performed to remove the portions of mask layer 40 from region 100. The respective step is illustrated as step 504 in the process flow shown in FIG. 12. Depending on the process and the distance between neighboring fins 126 (FIG. 1C), mask layer 40 may or may not have a residue portion left between neighboring fins 126, as shown in FIG. 2B. After mask layer 40 is removed, the portion of spacer layer 38 in region 100 is exposed, and an anisotropic etching is performed to etch spacer layer 38 in region 100, so that the top portions of spacer layer 38 on top of fins 126 are removed, exposing fins 126. The respective step is illustrated as step 506 in the process flow shown in FIG. 12. The remaining portions of spacer layer 38 on the sidewalls of gate stacks 28 become gate spacers 144 (FIG. 2C), and the remaining portions of spacer layer 38 on the sidewalls of fins 126 (FIG. 1B) become fin spacers 146 (FIG. 2B). The etching time of spacer layer 38 is selected so that fin spacers 146 have appropriate height H1 (FIG. 2B).

In a subsequent step, the exposed semiconductor fins 126 are recessed, for example, in an anisotropic or isotropic etching step, so that recesses 148 (FIGS. 2B and 2C) are formed to extend into semiconductor fins 126. The respective step is illustrated as step 508 in the process flow shown in FIG. 12. The bottoms of recesses 148 may be higher than, level with, or lower than the top surfaces of STI regions 22. The etching is performed using an etchant that attacks fins 126, and hardly attacks fin spacers 146. Accordingly, in the etching step, the height of fin spacers 146 is substantially not reduced. After the formation of recesses 148, photo resist 42 is removed, for example, in an ashing step.

FIGS. 3A, 3B, and 3C illustrate the patterning of spacer layer 38 in region 300. First, photo resist 50 is applied and patterned, wherein the photo resist 50 is illustrated in FIGS. 3B and 3C, and not in FIGS. 3A, although it still exists in FIG. 3A. Photo resist 50 may also be a single-layer photo resist or a tri-layer photo resist. The patterned photo resist 50 covers regions 100, 200, and 400, and leaves region 300 uncovered. Next, an etching step is performed to remove the portions of mask layer 40 in region 300. The respective step is illustrated as step 510 in the process flow shown in FIG. 12. Depending on the process and the distance between neighboring fins 326 (FIG. 2C), mask layer 40 may, or may not have a residue portion left between neighboring fins 326 (FIG. 3C). After mask layer 40 is removed, the portion of spacer layer 38 in region 300 is exposed, and an anisotropic etching is performed to etch spacer layer 38, so that the top portions of spacer layer 38 on top of fins 326 are removed, exposing fins 326. The respective step is illustrated as step 512 in the process flow shown in FIG. 12. The remaining portions of spacer layer 38 on the sidewalls of gate stacks 28 become gate spacers 344 (FIG. 3C), and the remaining portions of spacer layer 38 on the sidewalls of fins 326 (FIG. 3B) become fin spacers 346 (FIG. 3B). The etching time of spacer layer 38 is selected so that fin spacers 346 will have appropriate height H3 (FIG. 3B).

In a subsequent step, the exposed semiconductor fins 326 are recessed, for example, in an anisotropic or isotropic etching step, so that recesses 348 (FIGS. 3B and 3C) are formed to extend into semiconductor fins 326. The respective step is illustrated as step 514 in the process flow shown in FIG. 12. The bottoms of recesses 348 may be higher than, level with or lower than the top surfaces of STI regions 22. The etching is performed using an etchant that attacks fins 326, and hardly attacks fin spacers 346. Accordingly, in the etching step, the height of fin spacers 346 is substantially not reduced. After the formation of recesses 348, photo resist 50 is removed.

FIGS. 4A, 4B, and 4C illustrate the simultaneously epitaxy for forming epitaxy semiconductor regions 152 and 352 (which are source/drain regions of FinFETs) in regions 100 and 300, respectively. The respective step is illustrated as step 516 in the process flow shown in FIG. 12. In accordance with some embodiments of the present disclosure, the formation of epitaxy regions 152 and 352 includes epitaxially growing silicon phosphorous (SiP) or phosphorous-doped silicon carbon (SiCP), and the resulting FinFETs formed in regions 100 and 300 are n-type FinFETs. As shown in FIG. 4B, in the initial stage of the epitaxy, the grown epitaxy regions 152 and 352 are confined by fin spacers 146 and 346. After the epitaxy regions 152 and 352 are grown to higher than the top ends of epitaxy regions 152 and 352, respectively, lateral growth also occurs along with the vertical growth, and epitaxy regions 152 and 352 expand laterally.

The portions of epitaxy regions 152 grown from neighboring recesses 148 may be merged as a large epitaxy region, or remain separated from each other when the epitaxy if finished. The portions of epitaxy regions 352 grown from neighboring recesses 348 may also be merged into a large epitaxy region, or remain separated from each other when the epitaxy if finished. In addition, voids 153 and 353 may be formed when mergence occurs. Whether the mergence occurs or not depends on the heights of the respective fin spacers 146 and 346, and how long the epitaxy lasts. Accordingly, by adjusting heights H1 and H3 (FIG. 4B), one of the following four scenarios may occur: the mergence occurs for both epitaxy regions 152 and 352, the mergence occurs for epitaxy regions 152 but not for epitaxy regions 352, the mergence occurs for epitaxy regions 352 but not for epitaxy regions 152, and the mergence doesn't occur for either of epitaxy regions 152 and 352. FIG. 4D illustrates some exemplary embodiments, wherein the un-merged epitaxy regions 152 and 352 are illustrated if the corresponding epitaxy portions 608 in dashed lines don't exist.

Referring back to FIG. 4B, if, for example, it is desirable that the mergence occur for epitaxy regions 152 but not for epitaxy regions 352, fin spacers 146 may be formed to have height H1 smaller than height H3 of fin spacers 346. As a result, the lateral expansion occurs earlier for epitaxy regions 152 than epitaxy regions 352, and epitaxy regions 152 merge while epitaxy regions 352 don't merge. In accordance with some embodiments of the present disclosure, to make height H1 smaller than height H3, the period of time TP1 for etching spacer layer 38 (the step shown in FIG. 2B) may be selected to be longer than the period of time TP3 for etching spacer layer 38 (the step shown in FIG. 3B). In accordance with some embodiments of the present disclosure, ratio TP1/TP3 may be greater than about 1.5, and may be in the range between about 1.5 and about 5.0. As a result, height H3/H1 may be greater than about 1.5, and may be in the range between about 1.5 and about 5.0. With the merging of neighboring epitaxy regions, the resulting FinFET may have a higher drive (saturation current). With the neighboring epitaxy regions not merged, the resulting FinFETs may be more compact. Accordingly, different requirements of different circuits may be met at the same time without requiring the epitaxy regions to be formed by different epitaxy processes.

Converse to the above discussion, if it is desirable that the mergence occur for epitaxy regions 352 but not form epitaxy regions 152, fin spacers 146 may be formed to have height H1 greater than height H3 of fin spacers 346. In accordance with some embodiments of the present disclosure, ratio TP3/TP1 may be greater than about 1.5, and may be in the range between about 1.5 and about 5.0. Also, height H1/H3 may be greater than about 1.5, and may be in the range between about 1.5 and about 5.0.

In accordance with some embodiments, after the epitaxy, an implantation is performed to implant an n-type impurity such as phosphorous or arsenic into epitaxy regions 152 and 352 to form source/drain regions, which are also referred to using reference numerals 152 and 352. In accordance with alternative embodiments, no implantation of any n-type impurity is performed, and the n-type impurity was provided through the in-situ doping occurred during the epitaxy.

FIGS. 5A through 10C illustrate the formation of epitaxy regions for the FinFETs in regions 200 and 400, wherein the respective steps are similar to the repetition of the steps for forming epitaxy regions in device regions 100 and 300, except the conductivity types of some regions are inversed. First, an etching step is performed to remove remaining portions of mask layer 40 from regions 100, 200, 300, and 400. The respective step is illustrated as step 518 in the process flow shown in FIG. 12. Some residue portions of mask layer 40 may (or may not) be left after the etch. The resulting structure is shown in FIGS. 5A, 5B, and 5C.

Next, as shown in FIG. 6A, 6B, and 6C, mask layer 56 is formed. The respective step is illustrated as step 520 in the process flow shown in FIG. 12. The material and the formation methods of mask layer 56 may be selected from the same candidate materials and candidate methods for forming mask layer 40. For example, the material of mask layer 56 may be selected from silicon nitride, silicon oxide, SiCN, SiOCN, and SiON. Mask layer 56 may also be formed using ALD or CVD. The thickness of mask layer 56 may be in the range between about 2 nm and about 10 nm.

FIGS. 7A, 7B, and 7C illustrate the patterning of spacer layer 38 in region 200. First, photo resist 58 is applied and patterned, wherein the photo resist 58 is illustrated in FIGS. 7B and 7C, and not in FIGS. 7A, although it still exists in FIG. 7A. The patterned photo resist 58 covers regions 100, 300, and 400, and leaves region 200 uncovered. Next, an etching step is performed to remove the portions of mask layer 56 in region 200. Depending on the process and the distance between neighboring fins 226 (FIG. 2C), mask layer 56 may, or may not have a residue portion left between neighboring fins 226 (FIG. 7B). After mask layer 56 is removed, the portion of spacer layer 38 in region 200 is exposed, and an anisotropic etching is performed to etch spacer layer 38, so that the top portions of spacer layer 38 on top of fins 226 (FIG. 6B) are removed, hence exposing fins 226. The remaining portions of spacer layer 38 on the sidewalls of gate stacks 28 become gate spacers 244 (FIG. 7C), and the remaining portions of spacer layer 38 on the sidewalls of fins 226 (FIG. 7B) become fin spacers 246. The etching time of spacer layer 38 is selected so that fin spacers 246 have appropriate height H2 (FIG. 7B).

In a subsequent step, the exposed semiconductor fins 226 (FIG. 6B) are etched, for example, in an anisotropic or isotropic etching step, so that recesses 248 (FIGS. 7B and 7C) are formed to extend into semiconductor fins 226. The bottoms of recesses 248 may be higher than, level with, or lower than the top surfaces of STI regions 22. The etching is performed using an etchant that attacks fins 226, and hardly attacks fin spacers 246. Accordingly, in the etching step, the height H2 of fin spacers 246 is substantially not reduced. After the formation of recesses 248, photo resist 58 is removed.

FIGS. 8A, 8B, and 8C illustrate the patterning of spacer layer 38 in region 400. First, photo resist 62 is applied and patterned, wherein the photo resist 62 is illustrated in FIGS. 8B and 8C, and not in FIGS. 8A, although it still exists in FIG. 8A. The patterned photo resist 62 covers regions 100, 200, and 300, and leaves region 400 uncovered. Next, an etching step is performed to remove the portions of mask layer 56 in region 400. Depending on the process and the distance D2′ between neighboring fins 426 (FIG. 2C), mask layer 56 may, or may not have a residue portion left between neighboring fins 426 (FIG. 8B). After mask layer 56 is removed, the portion of spacer layer 38 in region 400 is exposed, and an anisotropic etching is performed to etch spacer layer 38, so that the top portions of spacer layer 38 on top of fins 426 (FIG. 7B) are removed, exposing fins 426. The remaining portions of spacer layer 38 on the sidewalls of gate stacks 28 become gate spacers 444 (FIG. 8C), and the remaining portions of spacer layer 38 on the sidewalls of fins 426 (FIG. 8B) become fin spacers 446. The etching time of spacer layer 38 is selected so that fin spacers 446 have appropriate height H4 (FIG. 8B).

In a subsequent step, the exposed semiconductor fins 426 (FIG. 7B) are etched, for example, in an anisotropic or isotropic etching step, so that recesses 448 (FIGS. 8B and 8C) are formed to extend into semiconductor fins 426. The bottoms of recesses 448 may be higher than, level with or lower than the top surfaces of STI regions 22. The etching is performed using an etchant that attacks fins 426, and hardly attack fin spacers 446. Accordingly, in the etching step, the height of fin spacers 446 is substantially not reduced. After the formation of recesses 448, photo resist 62 is removed.

FIGS. 9A, 9B, and 9C illustrate the simultaneously epitaxy for forming epitaxy regions 252 and 452 (which are source/drain regions of FinFETs) in regions 200 and 400, respectively. In accordance with some embodiments of the present disclosure, the formation of epitaxy regions 252 and 452 includes epitaxially growing silicon germanium, wherein boron may be in-situ doped, so that the resulting FinFETs formed in regions 200 and 400 are p-type FinFETs. As also shown in FIG. 9B, in the initial stage of the epitaxy, the grown epitaxy regions 252 and 452 are confined by fin spacers 246 and 446, respectively. After epitaxy regions 252 and 452 are grown to higher than the top ends of epitaxy regions 252 and 452, respectively, lateral growth also occurs along with the vertical growth, and epitaxy regions 252 and 452 expand laterally.

The portions of epitaxy regions 252 grown from neighboring recesses 248 may be merged into a large epitaxy region. In accordance with some embodiments of the present disclosure, the portions of epitaxy regions 452 grown from neighboring recesses 148 are not merged. This is achieved by making height H4 (FIG. 9B) of fin spacers 446 to be greater than height H2 of fin spacers 246. To make height H4 to be greater than height H2, the period of time TP4 for etching spacer layer 38 (the step shown in FIG. 8B) may be selected to be shorter than the period of time TP2 for etching spacer layer 38 (the step shown in FIG. 7B). In accordance with some embodiments of the present disclosure, ratio TP2/TP4 may be greater than about 1.5, and may be in the range between about 1.5 and about 5.0. As a result, height H4/H2 may be greater than about 1.5, and may be in the range between about 1.5 and about 5.0.

In accordance with alternative embodiments, the processes for forming fin spacers 236 and 446 may be adjusted to adjust heights H2 and H4, and to result in one of the following results: the mergence occurs for both epitaxy regions 252 and 452 (height H2 and H4 are substantially equal, for example, with difference smaller than about 10 percent), the mergence occurs for epitaxy regions 452 but not for epitaxy regions 252 (with height H2>H4), and the mergence doesn't occur for either of epitaxy regions 252 and 452.

In accordance with some embodiments, after the epitaxy, an implantation is performed to implant a p-type impurity such as boron or indium into epitaxy regions 252 and 452 to form source/drain regions, which are also referred to using reference numerals 252 and 452. In accordance with alternative embodiments, no implantation of p-type impurity is performed.

An etching step is then performed to remove remaining portions of mask layer 56 from regions 100, 200, 300, and 400, and FIGS. 10A, 10B, and 10C illustrate the structure after mask layer 56 is removed. N-type FinFET 166, p-type FinFET 266, n-type FinFET 366, and p-type FinFETs 466 are thus formed in regions 100, 200, 300, and 400, respectively. In subsequent steps, source/drain silicide regions (not shown) are formed on the top surfaces of source/drain regions 152, 252, 352, and 452. An Inter-Layer Dielectric (ILD, not shown) is formed to cover the illustrated FinFETs, and source/drain contact plugs (not shown) may be formed in the ILD to contact the source/drain silicide regions. Gate contact plugs (not shown) may also be formed to contact the illustrated gate electrodes in gate stacks 28. Also, the illustrate gate stacks 28 may be replaced with replacement gate stacks if gate stacks 28 are dummy gate stacks.

The embodiments of the present disclosure have some advantageous features. By separating the formation of fin spacers in different device regions, the heights of the fin spacers in different device regions may be adjusted separately. This advantageously results in the flexibility in forming merged or un-merged epitaxy source/drain regions. The formation of the fin spacers shares a common deposition process, and the epitaxy for forming source/drain regions for different device regions is also a common process. The manufacture cost is thus reduced.

In accordance with some embodiments of the present disclosure, a method includes forming a first gate stack extending on top surfaces and sidewalls of first semiconductor fins with the first semiconductor fins being parallel to and neighboring each other, forming a second gate stack extending on top surfaces and sidewalls of second semiconductor fins with the second semiconductor fins being parallel to and neighboring, each other, and forming a dielectric layer. The dielectric layer includes a first portion extending on the first gate stack and the first semiconductor fins, and a second portion extending on the second gate stack and the second semiconductor fins. In a first etching process, the first portion of the dielectric layer is etched to form first fin spacers on sidewalls of the first semiconductor fins. The first fin spacers have a first height. In a second etching process, the second portion of the dielectric layer is etched to form second fin spacers on sidewalls of the second semiconductor fins. The second fin spacers have a second height greater than the first height. The first semiconductor fins are recessed to form first recesses between the first fin spacers. The second semiconductor fins are recessed to form second recesses between the second fin spacers. The method further includes simultaneously growing first epitaxy semiconductor regions from the first recesses and second epitaxy semiconductor regions from the second recesses. The first epitaxy semiconductor regions grown from neighboring ones of the first recesses merge with each other. The second epitaxy semiconductor regions grown from neighboring ones of the second recesses are separate from each other.

In accordance with some embodiments of the present disclosure, a method includes etching a first semiconductor fin and a second semiconductor fin to form first recesses. The first and the second semiconductor fins have a first distance. A third semiconductor fin and a fourth semiconductor fin are etched to form second recesses. The third and the fourth semiconductor fins have a second distance equal to or smaller than the first distance. An epitaxy is performed to simultaneously grow first epitaxy semiconductor regions from the first recesses and second epitaxy semiconductor regions from the second recesses. The first epitaxy semiconductor regions are merged with each other, and the second epitaxy semiconductor regions are separated from each other.

In accordance with some embodiments of the present disclosure, a method includes, in a common deposition process, forming a dielectric layer including a first portion on top surfaces and sidewalls of first semiconductor fins and a second portion on top surfaces and sidewalls of second semiconductor fins. In separate etching processes, the first portion of the dielectric layer and the second portion of the dielectric layer are etched to form first fin spacers and second fin spacers, respectively. The first fin spacers have a first height, and the second fin spacers have a second height greater than the first height. The first semiconductor fins are etched to form first recesses between the first fin spacers. The second semiconductor fins are etched to form second recesses between the second fin spacers. In a common epitaxy process, first epitaxy semiconductor regions are growth from the first recesses, and second epitaxy semiconductor regions are grown from the second recesses. The first epitaxy semiconductor regions merge with each other, and the second epitaxy semiconductor regions are discrete from each other.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method comprising: forming a first semiconductor fin, a second semiconductor fin, a third semiconductor fin, and a fourth semiconductor fin protruding higher than top surfaces of isolation regions, wherein the first semiconductor fin is close to, and have a first distance from, the second semiconductor fin, and the third semiconductor fin is close to, and have a second distance from, the fourth semiconductor fin, wherein the second distance is equal to or smaller than the first distance; etching the first semiconductor fin and the second semiconductor fin to form first recesses; etching the third semiconductor fin and the fourth semiconductor fin to form second recesses; and performing an epitaxy to simultaneously grow first epitaxy semiconductor regions from the first recesses and second epitaxy semiconductor regions from the second recesses, wherein the first epitaxy semiconductor regions are merged with each other, and the second epitaxy semiconductor regions are separated from each other.
 2. The method of claim 1, wherein the second distance is smaller than the first distance.
 3. The method of claim 1, wherein the first recesses are between first fin spacers, and the second recesses are between second fin spacers higher than the first fin spacers.
 4. The method of claim 3 further comprising: forming a dielectric layer covering the first, the second, the third, and the fourth semiconductor fins; and etching the dielectric layer to form the first fin spacers and the second fin spacers.
 5. The method of claim 4, wherein the first fin spacers and the second fin spacers are formed in separate etching steps.
 6. The method of claim 4, wherein the first fin spacers and the second fin spacers are formed to have different heights.
 7. The method of claim 1, wherein the first epitaxy semiconductor regions and the second epitaxy semiconductor regions are source/drain regions of Fin Field-Effect Transistors (FinFETs).
 8. The method of claim 1, wherein the first epitaxy semiconductor regions and the second epitaxy semiconductor regions are p-type source/drain regions.
 9. The method of claim 1, wherein the first epitaxy semiconductor regions and the second epitaxy semiconductor regions are n-type source/drain regions.
 10. A method comprising: forming first gate spacers on sidewalls of a first semiconductor fin and a second semiconductor fin; forming first gate stacks on the first semiconductor fin and the second semiconductor fin; etching the first semiconductor fin and the second semiconductor fin to form first recesses; forming second gate spacers on sidewalls of a third semiconductor fin and a fourth semiconductor fin; forming second gate stacks on the third semiconductor fin and the fourth semiconductor fin; etching the third semiconductor fin and the fourth semiconductor fin to form second recesses; and performing an epitaxy to simultaneously grow first epitaxy semiconductor regions from the first recesses and second epitaxy semiconductor regions from the second recesses, wherein when the epitaxy is finished, the first epitaxy semiconductor regions are merged with each other, and the second epitaxy semiconductor regions are separated from each other.
 11. The method of claim 10, wherein the first and the second semiconductor fins have a first distance, and wherein the third and the fourth semiconductor fins have a second distance different from the first distance.
 12. The method of claim 11, wherein the second distance is smaller than the first distance.
 13. The method of claim 11, wherein the second distance is equal to the first distance.
 14. The method of claim 10 further comprising: forming first fin spacers on sidewalls of the first semiconductor fin and the second semiconductor fin; and forming second fin spacers on sidewalls of the third semiconductor fin and the fourth semiconductor fin, wherein the first fin spacers have a first height, and the second fin spacers have a second height different from the first height.
 15. The method of claim 14, wherein the first height is smaller than the second height.
 16. The method of claim 14, wherein the first fin spacers and the second fin spacers are formed in separate etching steps.
 17. A method comprising: forming a first semiconductor fin, a second semiconductor fin, a third semiconductor fin, and a fourth semiconductor fin protruding higher than top surfaces of isolation regions, wherein the first semiconductor fin is close to the second semiconductor fin, and the third semiconductor fin is close to the fourth semiconductor fin; forming gate stacks to cover portions of each of the first semiconductor fin, the second semiconductor fin, the third semiconductor fin, and the fourth semiconductor fin; in a first etching process, etching portions of the first semiconductor fin and the second semiconductor fin to form first recesses; in a second etching process, etching the third semiconductor fin and the fourth semiconductor fin to form second recesses; performing an epitaxy to grow first epitaxy semiconductor regions from the first recesses and second epitaxy semiconductor regions from the second recesses; and continuing the epitaxy until the first epitaxy semiconductor regions are merged with each other; and stopping the epitaxy before the second epitaxy semiconductor regions are merged with each other.
 18. The method of claim 17, wherein the first epitaxy semiconductor regions and the second epitaxy semiconductor regions are both n-type source/drain regions.
 19. The method of claim 17, wherein the first epitaxy semiconductor regions and the second epitaxy semiconductor regions are both p-type source/drain regions.
 20. The method of claim 17, wherein the first semiconductor fin and the second semiconductor fin have a first distance, and the third semiconductor fin and the fourth semiconductor fin are immediate neighboring fins having a second distance greater than the first distance. 